Plastic lenses are lightweight, not easily breakable and tintable as compared with inorganic lenses. Therefore, in recent years, plastic lenses have rapidly been in wide use in optical devices such as spectacle lenses and camera lenses.
It has become necessary for resins for plastic lenses to have more enhanced performances, and there have been demands for an increase in the refractive index, an increase in the Abbe number, a decrease in specific gravity, an increase in heat resistance, and the like. Thus, a variety resin materials for lenses have been hitherto developed and used.
Among others, optical materials formed from polythiourethane-based resins have high refractive index and high Abbe numbers, and are excellent in impact resistance, tintability, processability, and the like. Polythiourethane-based resins are obtained by allowing polythiols to react with polyiso(thio)cyanate compounds and the like.
It is required that in the case of being used in plastic lenses, polythiourethane-based resins be less color, have excellent resin colors, and be transparent. If the quality of polythiol is poor, the quality of the resin thus obtainable may also be poor.
The method for producing a polythiol is exemplified in the following patent documents.
Patent Document 1 or 2 describes a method of obtaining a particular polythiol compound by reacting 2-mercaptoethanol with epichlorohydrin, reacting the compound thus obtained with thiourea to obtain an isothiuronium salt, and then hydrolyzing the isothiuronium salt.
Patent Document 3 describes a method for producing a polythiol compound, the method including adjusting the amount of particular impurities that are contained in 2-mercaptoethanol to a predetermined range.
Patent Document 4 describes a method for producing a polythiol compound, the method including adjusting the content of calcium that is contained in thiourea to a predetermined range.
Patent Document 5 describes a method for producing a polythiol compound, the method including adjusting the content of calcium that is contained in thiourea, and the amount of particular impurities that are contained in 2-mercaptoethanol, to predetermined ranges.